Homogeneous Catalyst System Research Articles

Composition- and Condition-Dependent Kinetics of Homogeneous Ester Hydrogenation by a Mn-Based Catalyst.

The reaction medium and conditions are the key parameters defining the efficiency and performance of a homogeneous catalyst. In the state-of-the-art molecular descriptions of catalytic systems by density functional theory (DFT) calculations, the reaction medium is commonly reduced to an infinitely diluted ideal solution model. In this work, we carry out a detailed operando computational modeling analysis of the condition dependencies and nonideal solution effects on the mechanism and kinetics of a model ester hydrogenation reaction by a homogeneous Mn(I)-P,N catalyst. By combining DFT calculations, COSMO-RS solvent model, and the microkinetic modeling approach, the kinetic behavior of the multicomponent homogeneous catalyst system under realistic reaction conditions was investigated in detail. The effects of the reaction medium and its dynamic evolution in the course of the reaction were analyzed by comparing the results obtained for the model methyl acetate hydrogenation reaction in a THF solution and under solvent-free neat reaction conditions. The dynamic representations of the reaction medium give rise to strongly nonlinear effects in the kinetic models. The nonideal representation of the reaction medium results in pronounced condition dependencies of the computed energetics of the elementary reaction steps and the computed kinetic profiles but affects only slightly such experimentally accessible kinetic descriptors as the apparent activation energy and the degree of rate control.

Ester Hydrogenation with Bifunctional Metal-NHC Catalysts: Recent Advances.

Hydrogenation of ester to alcohol is an essential reaction in organic chemistry due to its importance in the production of a wide range of bulk and fine chemicals. There are a number of homogeneous and heterogeneous catalyst systems reported in the literature for this useful reaction. Mostly, phosphine-based bifunctional catalysts, owing to their ability to show metal–ligand cooperation during catalytic reactions, are extensively used in these reactions. However, phosphine-based catalysts are difficult to synthesize and are also highly air- and moisture-sensitive, restricting broad applications. In contrast, N-heterocyclic carbenes (NHCs) can be easily synthesized, and their steric and electronic attributes can be fine-tuned easily. In recent times, many phosphine ligands have been replaced by potent σ-donor NHCs, and the resulting bifunctional metal–ligand systems are proven to be very efficient in several important catalytic reactions. This mini-review focuses the recent advances mainly on bifunctional metal–NHC complexes utilized as (pre)catalysts in ester hydrogenation reactions.

Homogeneous Metal-Complex Catalyst Systems in the Partial Oxidation of Propane with Oxygen

Homogeneous Metal-Complex Catalyst Systems in the Partial Oxidation of Propane with Oxygen

Nontraditional Catalyst Supports in Surface Organometallic Chemistry

Surface organometallic chemistry (SOMC) as a field aims to blend the positive attributes of homogeneous and heterogeneous catalysis. Significant insight into heterogeneous systems has been gained over the years through the synthesis, characterization, and application of well-defined surface organometallic catalysts, predominantly supported on silica and alumina. Considerable research efforts have focused on the application of homogeneous methods to the synthesis and characterization of these systems. Homogeneous catalysis has thrived on its ability to electronically and sterically tune ligands to yield desired reactivity and selectivity. Efforts in SOMC, however, have only recently turned to harnessing the stereoelectronic diversity of potential inorganic support materials beyond silica and alumina in order to exert similar control on the reactivity of the organometallic active site. The support material is intrinsically linked to electronic structure and reactivity of heterogeneous organometallic systems in the same way that ligands exert control over homogeneous catalyst systems. The ability to tune the reactivity of heterogeneous catalysts by changing the support is of great value, and it is anticipated that this will represent an area of significant growth in the field. In this perspective, the use and future of non-traditional catalyst supports, such as sulfated metal oxides, modified silicas, and redox active supports is dis-cussed.

Batch and continuous esterification for the direct synthesis of high qualified biodiesel from waste cooking oils (WCO) with Amberlyst-15/Poly (vinyl alcohol) membrane as a bifunctional catalyst

Biodiesel, a mixture of fatty acids alkyl esters (FAAE), has recently received great concern as an excellent alternative to the petro-based diesel fuel. However, the production of biodiesel at commercial scale is obstructed by both cost of feedstock and the homogeneous catalyst system. To overcome these challenges, Amberlyst-15/Poly (vinyl alcohol) membrane (A-15/PVA) with catalytic and water-adsorbing dual functions have been prepared by a simple phase inversion method. The as-prepared A-15/PVA exhibited excellent catalytic activity and reusability in both batch and continuous esterification of acidified oil from waste cooking oil (WCO) with methanol for biodiesel production. The produced biodiesel exhibited high qualified properties and stability and can be directly applied in current diesel engines without modifying the engines. The exhaust of the biodiesel and biodiesel-blend could decrease the smoke opacity, carbon monoxide, hydrocarbon, while increase the nitrogen oxide emissions. The A-15/PVA demonstrated great potential in industrial production of biodiesel with high qualities from WCO.

Energy-Saving UHMW Polymeric Flow Aids: Catalyst and Polymerization Process Development

Crude oil and refinery products are transported worldwide to meet human energy needs. During transportation via pipeline, huge pumping power is required to overcome the frictional pressure drop and the associated drag along the pipeline. The reduction of both is of great interest to industry and academia. Highly expensive ultrahigh molecular weight (UHMW, MW a million Dalton) drag reducing polymers (DRPs) are currently used to address this problem. The present paper, therefore, emphasizes particularly the development of a high-performance catalyst system that synthesizes DRPs (using higher alpha-olefins)—a highly promising cost reduction alternative. This homogeneous catalyst system features a new concept that uses a cost-effective titanium-based Ziegler–Natta precatalyst and a cocatalystLewis base complex having both steric hindrance (around N heteroatom) and electronic effect. This novel work, which involves precatalyst–cocatalyst molecular separation and cocatalystmonophenyl amine association-dissociation phenomena, already generated several US patents. The subject catalyst prepares UHMW DRPs at room temperature, avoiding the use of zero and sub-zero temperatures. The resulting product almost tripled the rate of transportation of a selected grade of refinery product and saved about 50% pumping energy at ppm level pipeline concentration. It is also very easily soluble. Hence, massive modification of existing pipeline will be unnecessary. This will save additional infrastructure cost. This paper also summarizes challenges facing the development of improved heterogeneous catalysts, dispersed polymerization process, molecular simulation-based DRP product formulation, and model/theory of turbulent mixing and dispersion in the transportation pipeline setting.

Deoxydehydration of vicinal diols by homogeneous catalysts: a mechanistic overview.

Deoxydehydration (DODH) is an important reaction for the upconversion of biomass-derived polyols to commodity chemicals such as alkenes and dienes. DODH can be performed by a variety of early metal-oxo catalysts incorporating Re, Mo and V. The varying reduction methods used in the DODH catalytic cycle impact the product distribution, reaction mechanism and the overall yield of the reaction. This review surveys the reduction methods commonly used in homogeneous DODH catalyst systems and their impacts on yield and reaction conditions.

Triazole complexes of ruthenium immobilized on mesoporous silica as recyclable catalysts for octane oxidation

Triazole complexes of ruthenium immobilized on mesoporous silica materials were applied in the oxidation of n-octane. The reactions proceeded at a low catalyst loading of 0.01 mol% metal during which a maximum product yield of 13.8% was achieved. It was found that in all cases that the immobilized catalysts showed better activity when compared to the conventional homogeneous catalyst systems. More importantly, we were able to recycle the heterogenized catalysts while being employed in the oxidation of n-octane. During the recycling, only a slight drop in activity was observed.

Synthesis of Polylactide Nanocomposites Using an α-Zirconium Phosphate Nanosheet-Supported Zinc Catalyst via in Situ Polymerization

Immobilized catalysis integrates the advantages of both homogeneous and heterogeneous catalyst systems and thus has attracted high attention. Herein, we report the synthesis of a novel immobilized ...

Serendipity in Catalysis Research: Boron-Based Materials for Alkane Oxidative Dehydrogenation.

Light olefins such as ethylene and propylene form the foundation of the modern chemical industry, with yearly production volumes well into the hundreds of millions of metric tons. Currently, these light olefins are mainly produced via energy-intensive steam cracking. Alternatively, oxidative dehydrogenation (ODH) of light alkanes to produce olefins allows for lower operation temperatures and extended catalyst lifetimes, potentially leading to valuable process efficiencies. The potential benefits ofthis route have led to significant research interest due to the wide availability of natural gas from shale deposits. Advances in this area have still not yielded catalysts that are sufficiently selective to olefins for industrial implementation, and ODHstill remains a holy grail of selective alkane oxidation research. The main challenge in selective oxidation lies in preventing the overoxidation of the desired product, such as propylene during propane oxidation, to CO and CO2. Research into selective heterogeneous catalysts for the oxidative dehydrogenation of propane has led to the extensive use of vanadium oxide-based catalysts, and studies on the surface mechanism involved have been used to improve the catalytic activity of the material. Despite decades of research, however, selectivity toward propylene has not proven satisfactory at industrially relevant conversions. It is imperative for new catalytic systems that minimize product overoxidation to be developed for future applications of oxidative dehydrogenation processes. While rational catalyst design has been successful in developing homogeneous catalyst systems, its practical use in heterogeneous catalyst development remains modest. The complexity of surfaces with a variety of terminations and bulk structures, let alone their modification by the chemical potential of a reaction mixture, makes heterogeneous catalyst discovery serendipitous in many cases. The catalyst family presented in this Account is no exception. The importance of catalysis research lies in exploring the science behind serendipity. In this Account, we will first present our initial discovery of boron nitride (BN) as an unexpected catalyst for the oxidative dehydrogenation of light alkanes. Beyond its surprising activity, BN also drew interest due to its low selectivity to carbon oxides. This observation made BN distinct from previously studied metal oxide catalysts for selective alkane oxidation. We narrowed down its unique reactivity to the oxygen functionalization of the catalyst surface, particularly the formation of B-O species as probed by various spectroscopic techniques. In investigating the critical role of each of the structural elements during ODH, we discovered that not only BN but an entire class of boron-containing compounds are active and selective for the formation of propylene from propane. All these materials form a complex oxidized surface with a distribution of BO x surface sites. This discovery opens the doors to a new field of boron-based oxidation chemistry that currently has more questions than answers. We aim to make this Account a starting point for the research community to explore these new materials to understand their surface mechanisms and the surface species that offer a unique selectivity toward olefinic products. Effective use of these materials may lead to novel processes for efficient use of abundant light alkane resources by oxidation chemistry.

Interesterification optimization of waste cooking oil and ethyl acetate over homogeneous catalyst for biofuel production with engine validation

The interesterification of waste cooking oil (WCO) and ethyl acetate (ETA) are investigated in homogeneous catalyst system using sodium hydroxide (NaOH) and acetic acid (CH3COOH). Later on, the subsequent biofuel has been investigated for the combustion characteristics, gaseous and particulate matter related emissions in a single-cylinder agricultural diesel engine. In the reaction, fatty acid ethyl esters were the main product whereas triacetin by-product was formulated in a single phase biofuel. The parameters affected the free fatty acid (FFA) conversion were studied such as catalyst system, molar ratio of ETA:WCO, molar ratio of catalyst:WCO, reaction time, and temperature. The experimental results revealed that NaOH was more favorable than the acid catalyst counterpart. The 92% biofuel yield was reached from the optimization at the NaOH:WCO of 0.015:1 M ratio and the ETA:WCO of 30:1 M ratio at 80 °C in 3 h. In the engine validation at 3.4 and 6.6 bar IMEP loads, 1700 rpm speed, the results from the combustion analysis using an indicating system reveal that the WCO biofuel initiates the combustion faster with pronounce premixed combustion regime than that of diesel fuel. The specific fuel consumption of WCO biofuel was greater, leading to a slight reduction in brake thermal efficiency by 4% and 10% at 3.4 and 6.6 bar IMEP loads, respectively compared with diesel fuel. The nano-particle emissions was characterized by an electrical mobility spectrometer and analyzed in terms of particle number. The total particle number increased with smaller size when fueling with WCO biofuel. In comparison over the loads tested, the total particle number concentrations were in the ranges of 8.1 × 1011 to 1.1 × 1012 m−3 for WCO biofuel and 4.8 × 1011 to 5.7 × 1011 m−3 for diesel fuel. Meanwhile, the particle sizes were in the ranges of 182–251 nm for WCO biofuel and 279–402 nm for diesel fuel. Summarily, for the biofuel production, this homogeneous process is beneficial in terms of low-cost feedstock, glycerol-free and mild reaction condition.

An Effective Heterogeneous Copper Catalyst System for C–N Coupling and Its Application in the Preparation of 2-Methyl-4-methoxydiphenylamine (MMDPA)

A ligand-recyclable, environmentally benign heterogeneous catalyst system composed of CuI and polystyrene-supported N(-(4-(aminomethyl)naphthalen-1-yl)-N(-phenyl-1H-pyrrole-2-carbohydrazide (PSAP) has been established for Ullmann type C–N coupling based on the homogeneous catalyst system N′,N′-diphenyl-1H-pyrrole-2-carbohydrazide/CuI. This heterogeneous catalyst system maintained the catalytic effectiveness of the homogeneous catalyst. A variety of functionalized aryl bromides can be efficiently aminated with aryl amines and aliphatic amines with high selectivity for amines over alcohols. Moreover, a practical application of this catalyst system to promote the reaction of commercially available 4-methoxy-2-methylaniline and bromobenzene in 10 mmol scale, provided 2-methyl-4-methoxydiphenylamine (MMDPA) with 93% yield with the merit of the approach being simple operation for work-up and purification.

Kinetic investigations of styrene carbonate synthesis from styrene oxide and CO2 using a continuous flow tube-in-tube gas-liquid reactor

A continuous method of styrene carbonate (SC) synthesis from styrene oxide (SO) and carbon dioxide (CO2) is reported, using a semipermeable Teflon® AF-2400-based tube-in-tube gas-liquid reactor. The reaction was performed in the presence of and tetrabutylammonium bromide (TBAB) and zinc bromide (ZnBr2), acting as a highly efficient and economical homogeneous binary catalyst system. The exceptionally high permeation of CO2 through the membrane, combined with the high surface area to volume ratio in such continuous flow reactors resulted in complete SO conversion (100%) in a significantly reduced reaction time (tR) of 45 min at 120 °C and 6 bar of CO2 pressure. The effect of various operating conditions such as reaction temperature, CO2 pressure, residence time and catalyst molar ratio (ZnBr2/TBAB) was determined. Moreover, a detailed kinetic study of SC synthesis was carried out to determine the corresponding kinetic values (k and Ea) and activation parameters (ΔH‡, ΔS‡ and ΔG‡). Based on the experimental data, the reaction was found to be first-order in SO, TBAB and CO2 concentrations. The activation energy of SC synthesis in the presence of TBAB catalyst alone was calculated to be 55 kJ mol−1 over a temperature range of (90–120 °C), which was reduced to 32 kJ mol−1 as a result of using ZnBr2 as a co-catalyst.

Selective Decomposition of Cyclohexyl Hydroperoxide using Homogeneous and Heterogeneous CrVI Catalysts: Optimizing the Reaction by Evaluating the Reaction Mechanism

AbstractIn this study, known homogeneous and heterogeneous chromium(VI) catalyst systems were investigated with respect to the favored formation of cyclohexanone during the decomposition of cyclohexyl hydroperoxide (CHHP). The focus was on mechanistic studies using different spectroscopic methods as well as DFT calculations to further optimize the reaction conditions. As in previous decomposition studies, a mechanism via the formation of a metal alkylperoxido intermediate is probable. In situ spectroscopic studies revealed that in case of both the soluble and insoluble catalyst, the selective decomposition happens via a non‐radical, non‐redox mechanism at the CrVI stage through the formation of a cyclohexylperoxychromium(VI) complex. The proposed mechanism is supported by thorough DFT calculations.

Sixteen isostructural phosphonate metal-organic frameworks with controlled Lewis acidity and chemical stability for asymmetric catalysis

Heterogeneous catalysts typically lack the specific steric control and rational electronic tuning required for precise asymmetric catalysis. Here we demonstrate that a phosphonate metal–organic framework (MOF) platform that is robust enough to accommodate up to 16 different metal clusters, allowing for systematic tuning of Lewis acidity, catalytic activity and enantioselectivity. A total of 16 chiral porous MOFs, with the framework formula [M3L2(solvent)2] that have the same channel structures but different surface-isolated Lewis acid metal sites, are prepared from a single phosphono-carboxylate ligand of 1,1′-biphenol and 16 different metal ions. The phosphonate MOFs possessing tert-butyl-coated channels exhibited high thermal stability and good tolerances to boiling water, weak acid and base. The MOFs provide a versatile family of heterogeneous catalysts for asymmetric allylboration, propargylation, Friedel–Crafts alkylation and sulfoxidation with good to high enantioselectivity. In contrast, the homogeneous catalyst systems cannot catalyze the test reactions enantioselectively.

Ti alkoxide-based catalyst system in selective ethylene dimerization: High performance through modifying by alkylsilanes

ABSTRACTButene-1 production through a selective ethylene dimerization is one of the largest industrial processes, which is catalyzed by homogeneous catalyst system. The common industrial catalyst system is comprised of Ti alkoxide-based catalysts in combination with AlEt3 as an activator. In this study, the alkylsilanes were used as novel improving agents in the catalyst system for highly selective ethylene dimerization to butene-1. The nature and concentration of alkylsilanes on the dimerization rate, catalyst yield, by-product production, and selectivity to butene-1 were investigated in detail. It was found that alkylsilanes improved the productivity and selectivity of the catalyst. Moreover, the content of the solid by-product considerably decreased. The performance of the modified catalyst system was noticeably higher than that of the nonmodified one. Totally, it was proved that alkylsilanes could play a modifying role in the selective ethylene catalytic dimerization process.

A Transition-Metal-Free Homogeneous TEMPO-Based Catalyst: Aerobic Oxidation of Alcohols in Aqueous Media

In this paper the use of a novel TEMPO-based transition metal free homogeneous catalyst system, TEMPO/NaNO2/NH4Br/1,1,3,3-tetramethylguanidine sulfonic acid, for the aerobic oxidation of various alcohols to the corresponding aldehydes and ketones is described. The reactions are carried out in aqueous acetonitrile under atmospheric pressure and ambient temperature. The catalytic system exhibits excellent activity for the selective aerobic oxidation of alcohols.

Recyclable Acid–Base Bifunctional Core–Shell–Shell Nanosphere Catalyzed Synthesis of 5‐Aryl‐1H‐1,2,3‐triazoles through the “One‐Pot” Cyclization of Aldehydes, Nitromethane, and Sodium Azide

AbstractA magnetically separable core–shell–shell nanosphere, Fe3O4@nSiO2‐SO3H@MS‐NHCOCH3 (n=nonporous, MS=microporous SiO2), was fabricated as an acid–base collaborative catalyst for the three‐component cyclization of aromatic aldehydes, nitroalkane, and sodium azide to afford 1H‐1,2,3‐triazoles. The bifunctional heterogeneous catalyst showed high activity for this transformation and good chemoselectivity, and toxic HN3 was not released during the course of the reaction. A variety of aldehydes were transformed into the corresponding 5‐aryl‐1H‐1,2,3‐triazoles in up to 98 % yield. Furthermore, the catalyst could be recovered by using an external magnetic field and reused many times without any loss in activity. In contrast, a homogeneous catalyst system comprising ammonium acetate/acetic acid also worked in this three‐component cyclization to afford 1H‐1,2,3‐triazoles.

Second-Order Biomimicry: In Situ Oxidative Self-Processing Converts Copper(I)/Diamine Precursor into a Highly Active Aerobic Oxidation Catalyst.

A homogeneous Cu-based catalyst system consisting of [Cu(MeCN)4]PF6, N,N′-di-tert-butylethylenediamine (DBED), and p-(N,N-dimethylamino)pyridine (DMAP) mediates efficient aerobic oxidation of alcohols. Mechanistic study of this reaction shows that the catalyst undergoes an in situ oxidative self-processing step, resulting in conversion of DBED into a nitroxyl that serves as an efficient cocatalyst for aerobic alcohol oxidation. Insights into this behavior are gained from kinetic studies, which reveal an induction period at the beginning of the reaction that correlates with the oxidative self-processing step, EPR spectroscopic analysis of the catalytic reaction mixture, which shows the buildup of the organic nitroxyl species during steady state turnover, and independent synthesis of oxygenated DBED derivatives, which are shown to serve as effective cocatalysts and eliminate the induction period in the reaction. The overall mechanism bears considerable resemblance to enzymatic reactivity. Most notable is the “oxygenase”-type self-processing step that mirrors generation of catalytic cofactors in enzymes via post-translational modification of amino acid side chains. This higher-order function within a synthetic catalyst system presents new opportunities for the discovery and development of biomimetic catalysts.

Correlation of Microstructure, Rheological and Morphological Characteristics of Synthesized Polypropylene (PP) Reactor Blends Using Homogeneous Binary Metallocene Catalyst.

A novel binary homogeneous catalyst system based on (I): rac-Me2Si(2-Me-4-PhIn)2ZrCl2 and (II): (2-PhIn)2ZrCl2 catalysts at various molar ratios was utilized for the synthesis of polypropylene (PP) reactor blends with bimodal molecular weight distribution (MWD). The results of gel permeation chromatography analyses revealed that the catalyst (I) was responsible for the production of i-PP with high molecular weight (MW) while the individual use of catalyst (II) led to the production of an elastomeric PP with relatively low MW. However, application of the binary catalyst system led to high MW bimodal MWD products being highly dependent on the catalysts’ molar ratios. Increasing the molar ratio of catalyst (II) to catalyst (I) resulted in a notable enhancement of the products’ complex viscosity due to the increased MW, a higher level of chains’ entanglements and formation of amorphous blocks along the polymer chains. All products exhibited a single relaxation that shifted towards longer times upon changing the catalysts’ molar ratios. Scanning electron microscopy results revealed that the fracture surface of the blends, synthesized by the binary catalyst system, became more heterogeneous in comparison with the products obtained by the individual use of the catalyst (I). The observed heterogeneity was found to increase by increasing the amount of catalyst (II). Such morphological change was further corroborated by the dynamic rheological data, indicating a promising correlation between the linear rheological results and the morphological features of the synthesized PP reactor blends.